1. Field of the Invention
The present invention is directed generally to a zoom lens, and more particularly, to a zoom lens of an internal focalizing system for focusing by moving a part of an internal optical system.
2. Related Background Art
A dominant prior art focalizing system of a zoom lens is a so-called front lens focusing system in which a lens closest to an object is moved.
This front lens focusing system has such an advantage that photographing can be effected without causing a defocus even when executing a variable power operation in a state where a desired subject is focalized, because a moving quantity for focusing is invariable even by varying a magnifying power of the lens.
However, an entire length of the zoom lens is changed during focusing, and besides, the closest-to-object lens which is heavy and has a large effective diameter is moved. Therefore, a position of the center of gravity of the zoom lens changes, so that operability of the zoom lens is deteriorated. For this reason, quick focusing is difficult. Besides, particularly in a camera incorporating an auto-focusing (AF) device, a load exerted on a driving source for focusing increases. A respondency of auto-focusing remarkably declines. There also arises a problem in which the driving source for focusing increases in size to cope with this.
The following is an additional problem. Fluctuations in aberration on the occasion of focusing are relatively large when seeing the front lens focusing system in terms of an aberration of the lens. Especially, fluctuations in spherical aberration and in astigmatism tend to be large.